{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"d81ba192ab849211f0adcdd807496785d40e8d7203b2ee2a84ce5e00ce536a07","created":"2026-10-03T07:18:04Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"dd4a323142b8ee21e62e29f42aec3cd12dc2caddb8c47ce5b30f0eb793f981b1","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"classification","assisted_by":[],"external_id":"grav.bh-interior.firewall-old-horizon","kind":"well-posed","literature_status":"contested","n":"1","parents":[],"plain":"A 2012 argument showed that information preservation, normal physics outside the black hole, and a smooth horizon cannot all hold once a black hole has emitted half its entropy. Which assumption fails is still debated.","posed_since":"2012","precise":"For a black hole past the Page time (when half the initial Bekenstein-Hawking entropy has been radiated), determine whether a freely falling observer crossing the horizon finds the local vacuum (mode occupation near zero for wavelengths much shorter than $r_s = 2GM/c^{2}$) or high-energy quanta up to the cutoff (a firewall). An answer specifies which assumption of the Almheiri-Marolf-Polchinski-Sully argument fails (unitary evaporation, local effective field theory outside the stretched horizon, a black hole that a distant observer describes as a quantum system with $\\exp(S_{\\mathrm{BH}})$ states, or a smooth horizon for an infalling observer), in a framework where the computation is controlled.","problem_ref":null,"references":"","settled_by":"A calculation in a UV-complete theory, such as an evaporating AdS black hole with a known dual, of infalling observables at the horizon after the Page time.","status_note":"Entanglement-wedge reconstruction (2019-2020) supports a smooth horizon with the interior encoded in the radiation, but no derivation in a UV-complete theory settles it as of 2026.","title":"Does an infalling observer see vacuum at an old black hole's 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